FORT CARSON, Colo.—Heavy combat in Iraq where U.S. soldiers witnessed the deaths of many fellow troops may have contributed to hostile behavior by Colorado-based veterans who have been accused or convicted in 11 slayings back home, the Army said Wednesday.

The Army launched an investigation after soldiers from the 4th Brigade Combat Team, 4th Infantry Division were accused in a spate of five killings around Colorado Springs, home to Fort Carson, in 2007 and 2008.

The combat team had nicknamed itself the Lethal Warriors.

Six other slayings involving unit soldiers occurred in Colorado and other states since 2005.

Nationally, at least 121 Iraq and Afghanistan war veterans have committed a killing in the United States or been charged in one.

Army investigators compared the Fort Carson unit of about 3,700 soldiers with a similarly sized unit and found it suffered more combat deaths in Iraq and was deployed there longer.

The task force report released Wednesday said more study is needed on the links between combat and aggressive behavior. It suggested the Army find a way to identify soldiers who have been exposed to fierce combat.

Investigators focused on the cases of 14 soldiers accused of murder, manslaughter, attempted murder and aggravated assault, mostly with firearms. They found the accused had experienced heavy combat in Iraq and that half of those interviewed reported witnessing war crimes, including the killing of civilians.

Back home, the soldiers carried weapons with them because they felt “naked” and unsafe and had difficulty transitioning to civilian life. Some said they felt “weird” and didn’t fit in, the Army report said.

“There, we were the law; here, the cops are the law,” one of the accused told investigators.

The Army report says the accused claimed their commanders and fellow soldiers did not encourage them to seek help at home.